kcbhiw (another miata autocrosser) fabbed it up for me. It's just a 2.25" dynomax muffler from summit with a 2.25" flange and a 45* mandrel bend from ebay.
Total cost? $70 max if you have a friend who can do some basic fabrication.
If you look at the muffler itself, it's basically a 2.25" straight pipe with a buttload of holes drilled through it. There's some sound deadening around the outside, and then a casing. If you look through it, it's just a 2.25" pipe. It's fantabulous. And it has improved performance. The biggest thing I noticed was a significant improvement on throttle tip / general throttle response in compared to the supertrapp I used to have. I don't know how much more power it makes yet (Dyno time scheduled for early april), but it sure works well.
